Re: [PATCH] vhost: Fix bad return of descriptors to SVQ

[Eugenio Perez Martin]

On Fri, Apr 1, 2022 at 4:30 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Fri, Apr 1, 2022 at 2:14 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> >
> > Only the first one of them were properly enqueued back.
> >
> > Fixes: 100890f7ca ("vhost: Shadow virtqueue buffers forwarding")
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >  hw/virtio/vhost-shadow-virtqueue.c | 17 +++++++++++++++--
> >  1 file changed, 15 insertions(+), 2 deletions(-)
> >
> > diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> > b/hw/virtio/vhost-shadow-virtqueue.c
> > index b232803d1b..c17506df20 100644
> > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > @@ -333,13 +333,25 @@ static void 
> > vhost_svq_disable_notification(VhostShadowVirtqueue *svq)
> >      svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> >  }
> >
> > +static uint16_t vhost_svq_last_desc_of_chain(VhostShadowVirtqueue *svq,
> > +                                             uint16_t i)
> > +{
> > +    vring_desc_t *descs = svq->vring.desc;
> > +
> > +    while (le16_to_cpu(descs[i].flags) & VRING_DESC_F_NEXT) {
> > +        i = le16_to_cpu(descs[i].next);
>
>
> This seems to be a guest trigger-able infinite loop?
>

This is the list of the SVQ vring. We could consider an infinite loop
triggable by the device if it can write the vring directly.

I can add a counter in the loop, or to maintain an internal copy of
the vring so it's completely hardened against malicious/bad devices.
It should be done for packed vring anyway.

Thanks!

> Thanks
>
>
> > +    }
> > +
> > +    return i;
> > +}
> > +
> >  static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
> >                                             uint32_t *len)
> >  {
> >      vring_desc_t *descs = svq->vring.desc;
> >      const vring_used_t *used = svq->vring.used;
> >      vring_used_elem_t used_elem;
> > -    uint16_t last_used;
> > +    uint16_t last_used, last_used_chain;
> >
> >      if (!vhost_svq_more_used(svq)) {
> >          return NULL;
> > @@ -365,7 +377,8 @@ static VirtQueueElement 
> > *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
> >          return NULL;
> >      }
> >
> > -    descs[used_elem.id].next = svq->free_head;
> > +    last_used_chain = vhost_svq_last_desc_of_chain(svq, used_elem.id);
> > +    descs[last_used_chain].next = svq->free_head;
> >      svq->free_head = used_elem.id;
> >
> >      *len = used_elem.len;
> > --
> > 2.27.0
> >
>